Xylene isomers find wide and varied application. They are especially valuable as intermediates in chemical processes. By way of example, para-xylene is a feedstock for terephthalic acid, which finds use in the manufacture of polyester fibers and films, meta-xylene is used in the manufacture of dyes, and ortho-xylene is used as a feedstock for phthalic anhydride, which finds use in the manufacture of plasticizers. Para-xylene is currently the most valuable of the xylene isomers and, although research related to obtaining (e.g., producing or purifying) para-xylene is voluminous, there is still intensive research in the area.
There are many possible feeds currently used to obtain para-xylene. The majority of para-xylene produced today comes from catalytic reforming, which involves dehydrogenation and dehydrocyclization of naphtha feedstocks. The effluent of the reforming process, known as reformate, is rich in aromatics, particularly benzene, toluene, and mixed xylenes (BTX), and is used as feedstock to aromatics plants. Processes exist to increase the yield of para-xylene over the equilibrium mixture in the reformate, including selective toluene disproportionation and selective methylation of benzene and/or toluene with methanol.
Recently, significant research has focused on finding alternative sources and methods for producing BTX and particularly para-xylene. For example, although steam cracking, or pyrolysis, is the preferred method of producing light olefins (ethylene, propylene, and butenes) from heavier hydrocarbon feedstocks, the process also generates a by-product termed pyrolysis gasoline, steam cracked naphtha (SCN) or pygas. Pygas is a complex mixture of C6 to C10+ hydrocarbons that is rich in aromatics, particularly benzene and toluene, but also contains C8, C9, and C10+ aromatics. Similarly, catalytic cracking, particularly fluid catalytic cracking (FCC), in addition to producing fuels and light olefins, generates a C6 to C10+ aromatic rich stream which is similar to pygas and is generally known as cat naphtha. These processes also produce C4 and C5 olefinic streams (containing di-olefins and acetylenes) which have some utility but tend to be of lower value than aromatic products and lighter olefins (ethylene and propylene). There is, therefore, significant interest in developing methods of upgrading alternate feed sources, such as pygas and cat naphtha, to increase the yield of ethylene, propylene, BTX; and preferably para-xylene and propylene. There are some processes proposed to upgrade these streams to produce BTX but they consume expensive hydrogen and co-produce lower value light saturates rather than higher value light olefins.
U.S. Pat. No. 7,176,339 discloses a process for producing xylenes from reformate, which process comprises: (a) providing a reformate containing hydrogen, C1 to C5 hydrocarbons, C6 to C7 hydrocarbons comprising benzene, toluene or mixtures thereof, and C8+ hydrocarbons; (b) removing at least a portion of said hydrogen from said reformate to produce a product containing C6 to C7 hydrocarbons comprising benzene, toluene, or mixtures thereof, and C8+ hydrocarbons; and (c) methylating at least a portion of the benzene, toluene, or mixtures thereof present in said product with a methylating agent under vapor phase conditions and in the presence of a catalyst effective for the methylation to produce a resulting product having a higher para-xylene content than the reformate. The catalyst comprises a zeolite-bound-zeolite catalyst and/or a selectivated zeolite and the zeolite comprises ZSM-5. A similar process is disclosed in U.S. Pat. No. 7,629,498.
U.S. Pat. No. 7,563,358 discloses process for producing BTX-enriched product from a hydrocarbon feed comprising: (a) C6+ non-aromatic cyclic hydrocarbons; (b) C8+ single-ring aromatic hydrocarbons having at least one alkyl group containing two or more carbon atoms; and (c) C9+A single-ring aromatic hydrocarbons having at least three methyl groups. According to the process, the feed is contacted, in the presence of hydrogen, with a catalyst comprising at least one Group VIII metal and a large or intermediate pore molecular sieve having an alpha value, before incorporation of the Group VIII metal, from about 2 to less than 100 under conditions sufficient for (i) forming aromatic hydrocarbons from C6+ non-aromatic cyclic hydrocarbons; (ii) dealkylating C8+ single-ring aromatic hydrocarbons having at least one alkyl group containing two or more carbon atoms; (iii) transalkylating C9+ single-ring aromatic hydrocarbons having at least three methyl groups; and (iv) disproportionating toluene, to produce a product containing an increased amount of BTX compared to the feed. A preferred hydrocarbon feed is steam cracked naphtha.
Given the higher value of xylenes, especially para-xylene, than toluene and benzene, there is a desire to convert a portion of the toluene and benzene produced in the above petrochemical processes to xylenes, especially para-xylene. The present invention provides an effective process for doing so.